its as simple as removing all the screws on the back and carefully(make sure you get all the screws, a broken card would suck) taking the heatsink off(there will be some resistance as the heatsink thermal paste will have stuck to the gpu....so take it slow...)
As said any after marker cooler tells you just how to take it off..
Also make sure you have heatsinks for the ram(not sure how much effect is has since my OCZ rams spreaders are not even touching all the chips and it still works fine...but video card ram is faster...) and any other warm components like the voltage regulation system. this should be in the after market instructions as well.
With the 8800 heatsinks not being loud i would not even bother replacing it unless you want to over clock to the moon. thats another story(with the fact that the core and shaders are linked....so you have to be careful..and maybe even flash the card....danger 8800....danger....)